The invention relates to an intraluminal implant (stent) for dilating a blood vessel, as recited in the preamble of claim 1
Intraluminal implants for dilating blood vessels having a first diameter and being adapted to be expanded so as to have a second diameter once placed inside the blood vessel have been known since 1978. The oldest implants are simple helically wound wire coils which are introduced in a radially compressed condition and released when in place at the desired location in the lumen so that they can expand radially and, at the same time, push the wall of the vessel radially outwardly. The application of spring-elastic stents was replaced by plastically deformable implants as balloon-tipped catheters became more widely used.
There are essentially three basic types of these implants. Structures having a wound endless wire are known from EP-A-0 282 175, EP-A-0 312 852, EP-A-0 378 151 or U.S. Pat. No. 5,133,732, U.S. Pat. No. 5,135,536 and U.S. Pat. No. 5,562,697 or DE-A-44 32 938, just to name a few. These stents do not have a very stable shape and, therefore, are difficult to mount on a balloon-tipped catheter.
By far the greatest number of intraluminal implants consist of a kind of braided fabric made from a plurality of wires of extremely complicated form and partly reinforced by spot welding. A selection of such stents may be gathered from EP-A-0 421 729, EP-A-0 423 916, EP-A-0 480 667, EP-A-0 565 251 or EP-A-0 744 164, just to name a few. From the point of view of manufacture, these implants are highly complicated and, consequently, also very expensive.
The stent of interest here is a plastically deformable intraluminal implant which is cut from a thin-walled tube by means of a laser so as to present a kind of mesh structure. It is a great advantage of such implants that their surface is an absolutely smooth cylinder when they are not in the widened or expanded state and, therefore, they can be mounted easily on a balloon-tipped catheter and they do not cause frictional drag worth mentioning when being introduced and displaced inside a vessel.
The remaining flat webs have a thickness which is much less than that of wires for braided stents of comparable diameter. The overall diameter of the implant thus is reduced accordingly and that permits its use in vessels having a small inner diameter or lumen.
A first embodiment according to EP-B-0 221 570 shows an implant consisting of a piece of tube in which rectangular recesses, all extending in axial direction, are formed by lasering. When not expanded, such an implant practically is flexurally stiff. As a consequence, the length of the stent is limited and that may make it necessary to place several stents next to one another. It is a delicate matter to position them so that they will not overlap and yet be almost contiguous. For this reason, a more advanced solution provides for the basically similarly shaped stent, to consist of a plurality of sections integrally joined each by a single straight web, such as specified in U.S. Pat. No. 5,195,984 from which the pre-characterizing part of claim 1 starts. The flexibility of this solution corresponds to a coarse link chain and has a tendency to buckle.
All the known intraluminal implants made from a tube suffer contraction in axial direction when their diameter is widened. This contraction is difficult to estimate although it is directly related to the expansion. That makes it difficult for the physician to make the right choice of the stent needed and to position it correctly.
This holds also true for a conventional stent (WO 97/26689) comprising several rings of meandering webs of material. The outermost rings of, e. g., six adjacent rings may be longer than the inner rings in between. The rings are interconnected each by three straight webs extending obliquely with regard to the center axis of the rings. The rings are offset in angular direction to each other by 120xc2x0. Another similarly formed stent (DE 44 18 336 A1) has provided, parallel to the axis, short interconnecting webs between adjacent rings. Finally, a stent is known (DE 297 02 671) the rings of which consist of a plurality of adjacent, hair-pin shaped strands which form a tightly meandering web of material. The rings are interconnected by a plurality of web ogives having peak and bottom. All these known stents have a reduced axial flexibility. When expanding the stents, there is a pronounced reduction of the total length, because the interconnecting webs between the rings are not or practically not stretched. The axial or frictional force needed therefor cannot be exerted by a balloon-tipped catheter during expansion.
It is the object of the invention to improve a luminal implant (stent) of the kind mentioned just above which after insertion into a vessel is plastically expanded in the radial direction, such that it is flexible in axial direction in the non-expanded state and which, however, does not kink or upset and does not display any relevant change in length when being expanded. After expansion, it should possess sufficient elastic properties so as to conform to the deformations of the coronary vessels during heartbeat.
This object is met by an implant with the characterizing features of claim 1. Further advantageous modifications may be gathered from the dependent claims.